Optical document scanners for reading, by mark sensing, data recorded on a document are well-known. Typically, such scanners comprise a camera containing a linear array of photosensitive elements coupled to a signal processor associated with the scanner. The camera electronically scans marks made on the documents, usually pencil marks, by receiving light reflected from the document when it is illuminated. Typically, the pencil marks represent responses to multiple choice questions or other information. The photosensitive elements of the camera define pixels and provide electrical signals which represent the placement of the marks on the document. These signals are supplied to the signal processor.
The aforementioned type of documents generally contain marking positions lightly preprinted thereon for recording the responses (pencil marks). The marking positions may take one or more of a variety of formats (e.g., circular or elliptical) and are usually aligned in rows parallel to the width of the document. Each document is transported through the scanner in a direction perpendicular to the rows so that each row is successively indexed through a scanning location. At the scanning location, a light source illuminates the entire row so that the data, i.e., the placement of the pencil marks, can be read by the camera. A guide within the scanner maintains document registration as it is transported therethrough. Timing or guide marks are generally printed along a longitudinal edge of the document to signal to the camera the arrival of each row of marking positions at the scanning location. FIG. 1 illustrates a typical prior art scanner of the aforementioned type. As illustrated, such scanner comprises a camera 1 coupled to a signal processor 9, a light source 2 and a guide 3. Document 4 has a plurality of marking positions 5 for placement of pencil marks 6. The document 4 also has timing marks 7 and is transported in the direction indicated by the arrow 8.
The prior art is typified by U.S. Pat. No. 4,300,123 ("the '123 patent"), which has been commercially embodied as the Model W-201 scanner by Westinghouse Electric Company. This machine is for particular use with documents having an array of preprinted elliptically shaped marking positions for recording information by pencil marks. The Westinghouse machine also employs a Fairchild Company Charge Coupled Device ("CCD") Model 1310 electronic camera having a plurality of square photosensitive elements. It will be apparent that, since the photosensitive elements are square and the marking positions are elliptical, no side of any marking position can be precisely aligned with any photosensitive element during scanning. Precise scanning and accurate reading of the recorded data can, therefore, be problematic.
The prior art is also typified by the Model NCS 22-85 optical document scanner manufactured by National Computer Systems. This machine has particular application to documents having an array of circularly shaped marking positions. This format is acceptable if the scanner reading the document employs individual phototransistors. However, if the scanner employs a camera having square photosensitive elements, such as the Fairchild 1310 camera, this format is inadequate for precise scanning for the same reasons as discussed above. These limitations of the prior art are discussed in more detail below.
Another limitation of the prior art is the inability to automatically adjust for lateral displacement of the document while it is transported through the scanner. The above-described model NC-22-85 machine does not provide any means for automatically correcting for lateral displacement of the document. The above-described described '123 patent, however, does describe a means for automatically adjusting the scan to correct for minor lateral displacement of the document. According to the '123 patent, it is said that lateral displacement is measured in terms of pixel counts which indicate the actual transverse location of the document with respect to the array of photosensitive elements in the camera. But, these pixel counts correspond to only one edge of the document, and do not and cannot provide an indication of variations in the width of the document. This is a problem inasmuch as actual document width can vary for a number of reasons.
Thus, there are two serious limitations of prior art scanners: (1) imprecise registration of a raster generate square photosensitive elements of the camera with circular or elliptical marking positions; and, (2) uncompensated hygrometric and other variations in the width of documents scanned. These and other limitations of the prior art are addressed by the present invention. A greater appreciation of the instant invention will be realized, however, by a more detailed analysis of the problems of prior art scanners.
Underscanning or overscanning of marking positions unavoidably results when using a commercially available camera to scan the above-described documents having circular or elliptical marking positions. It is not possible to scan an entire marking position without also scanning some of the white background of the document because the elements of the photosensitive array are square. Either part of the marking position will not be scanned (underscanning), or part of the white background of the document around the marking position will be scanned (overscanning). Both results are undesirable. In both underscanning and overscanning of a marking position, the sum of the intensity values of the camera's exposed photosensitive elements does not represent the true intensity of the light reflected by the pencil marks. The discrepancies between the true intensities and the intensities perceived by the camera are often great enough to seriously impair the discrimination of faint, albeit intended, marks as compared to dark erasures.
The foregoing is made evident by the following. In the case of underscanning, the proportion of the area of a circular marking position left unscanned by the largest possible square raster within the marking position (see FIG. 2) is: EQU (.pi.R.sup.2 -2R.sup.2)/.pi.R.sup.2 =1 -2/.pi.=0.36
where R=radius of circular marking position.
Thus, in the case of underscanning, the minimum unscanned area of a circular marking position by a square photosensitive element is 36% of the total surface area. In the case of overscanning, the proportion of the area of a circular marking position which is overscanned (on the white background of the document) by the smallest possible square raster which just encloses the marking position (see FIG. 3) is: EQU (4R.sup.2 -.pi.R.sup.2)/.pi.R.sup.2 =4/.pi.-1=0.27
where R=radius of circular marking position.
Thus, in the case of overscanning, 27% additional area is scanned relative to the area enclosed by the circular marking position.
Both of the above-described Model W-201 and Model NCS 2285 machines are for use with so-called "long-grain" paper documents, i.e., the fibers of the paper are essentially parallel to the long dimension of the document. The documents for these scanners typically measure 81/2 by 11 inches and are generally transported in a direction parallel to their long dimension. Long-grain orientation is used to impart greater stiffness to the paper in the direction in which the sheets are transported under the scanning head. The greater stiffness decreases the likelihood of paper jams in the transport mechanism of the scanner. A problem with long-grain orientation of the paper fibers, however, is that it increases the vulnerability of the sheets to hygrometric variations in their width. The humidity content affects the size of a sheet of paper mostly in the direction perpendicular to the fibers, i.e., width variations may occur. In extreme cases the width of an 81/2" by 11" document may shrink or expand by as much as 0.04". It will be appreciated variations in width may impair the registration of the marking positions relative to the "electronic mask" or scanning windows in the scanner designed to correlate the photosensitive elements to the marking positions for the camera. This phenomenon is described in the aforementioned '123 patent. In extreme, but not uncommon, cases of this impairment, misregistration renders accurate scanning of high density marking positions virtually impossible. No common humidity treatment can be applied to the documents at the time of scanning to correct for width discrepancies because different batches of answer sheets are unavoidably subjected to different histories of humidity conditions.
Another important limitation of prior art scanners is that their cameras do not provide sufficient detail for effective pictorial scanning. The aforementioned Fairchild Model 1310 camera has only 1024 photosensitive elements. For an eight-inch scanning line, this provides only 1024/(8.times.72)=1.77 pixels per printer's point. (As is known, the standard of measure is 72 printer's points per inch.) This is little more than half the detail of the three pixels per printer's point of the commonly accepted facsimile standard.
In view of the foregoing limitations of the prior art, it will be appreciated that it is desirable to provide a scanning and mark sensing method and apparatus that: (1) provides proper registration of the marking positions with respect to the array of photosensitive elements of the camera; and, (2) compensates, automatically, for hygrometric and other variations in the width of documents as they pass through the scanning location. The instant invention achieves these and other goals.